I tried to recreate this and the app still seemed to be functional. Did you make any app changes besides what's stated in the README.md?
Did you have errors on git push?
Were there errors in the nodejs log?
Was you app in a running state when you were getting the 503?

I tried to recreate this in STG(devenv-stage_357),still returns 503,but my app's status was started.
I did not make any app changes,only followed the README.md.
There were some errors in node.log,please refer to #attachment 1[details]
And I also tried to recreate this in the latest devenv(devenv_3319),but app was ok.I could access the app's url successfully.But in node.log,there were some errors,please refer to #attachment 2[details]

I get this problem using the upstream https://github.com/mongolab/mongolab-openshift-quickstart because they haven't accepted our V2 compatability pull request yet.
This works just fine with our fork which does have the V2 changes. I will attempt to contact somebody at MongoLabs to try to get them to accept the PR.